1. Technical Field of the Invention
The present invention relates generally to the field of mobile telecommunications and, in particular, to a system for processing multiple random access mobile-originated calls.
2. Description of Related Art
The next generation of mobile communications systems will be required to provide a broad selection of telecommunications services including digital voice, video and data in packet and channel-switched modes. As a result, the number of calls being made is expected to increase significantly, which will result in much higher traffic density on random access channels (RACHs). Unfortunately, this higher traffic density will also result in increased collisions and access failures. Consequently, the new generation of mobile communications systems will have to use much faster random access procedures, in order to increase their access success rates and reduce their access request processing times.
In most mobile communications systems, such as, for example, the European joint development referred to as the "Code Division Testbed" (CODIT), and systems operating in accordance with the IS-95 Standard (ANSI J-STD-008), a mobile station can gain access to a base station by first determining that the RACH is available for use. Then, the mobile station transmits a series of access request preambles (e.g., single 127 chip symbols) with increasing power levels, until the base station detects the access request. In response, the base station starts the process of controlling the mobile station's transmitted power via a downlink channel. Once the initial "handshaking" between the mobile station and base station has been completed, the mobile user transmits a random access message.
More specifically, in a CODIT-based Code Division Multiple Access (CDMA) system, a mobile station will attempt to access the base station receiver by using a "power ramping" process that increases the power level of each successive transmitted preamble symbol. As soon as an access request preamble is detected, the base station activates a closed loop power control circuit, which functions to control the mobile station's transmitted power level in order to keep the received signal power from the mobile station at a desired level. The mobile station then transmits its specific access request data. The base station's receiver "despreads" the received (spread spectrum) signals using a matched filter, and diversity-combines the despread signals to take advantage of channel multipath diversity.
In an IS-95 CDMA system, a similar random access technique is used. However, the primary difference between the CODIT and IS-95 process is that the IS-95 mobile station transmits a complete random access packet. If the base station does not acknowledge the access request, the IS-95 mobile station re-transmits the access request packet at a higher power level. This process continues until the base station acknowledges the access request.
In the Spread Spectrum Slot Reservation Multiple Access (SS-SRMA) System, a slotted ALOHA random access scheme is used. Generally, a mobile station transmits and re-transmits the random access packet until the correct reception of the random access packet is acknowledged by the base station. The transmissions are spaced apart by random time intervals. However, the slotted ALOHA random access process is inherently unstable. Consequently, some form of feedback loop must be implemented to stabilize such a system. Furthermore, no provision is made to distinguish between multiple signal arrivals, which results in an increasing number of collisions and, necessarily, re-transmissions, and thus worsens the instability problem.
A number of significant problems exist with the above-described random access techniques. For example, in a CODIT system, a base station receiver can detect only one random access at a time. If two mobile stations transmit their access requests at the same time, either the two messages will collide and be destroyed, or one message will be recognized and the other ignored. Consequently, the throughput efficiency of such a system is relatively low, its collision ratio is high, and the mean time that it takes to achieve a successful access is excessively long. A system that would resolve collisions problems and effectively process multiple random requests is desirable, but to increase the throughput of the existing systems would require an increase in the number of access codes used (which are typically limited in number), and a corresponding number of additional matched filters would be required at the base station receiver.
Although existing IS-95 and SS-SRMA documents indicate that their base station CDMA receivers can demodulate multiple random access messages arriving in the same slot, these documents do not specify how this process can be implemented. Furthermore, the IS-95 documents do not indicate how to control the transmission power of the individual multiple access signals.
Additionally, the above-described IS-95 and CODIT random access processes are relatively slow, because numerous power ramping increments have to occur until an access request can be detected and acknowledged, and re-transmissions in the SS-SRMA system cause intolerable delays. Also, it is a relatively difficult process to implement a system that can receive multiple random access requests and control the power level of each individual request message. Consequently, for all of the above-described reasons, the usage efficiencies of the CODIT, IS-95 and SS-SRMA RACHs are very low, and traffic interference experienced by users of these systems is excessive due to this inefficient use of the RACH and the numerous re-transmissions required.
Yet another problem with these CDMA systems is that they are basically not designed to resolve packet collision problems. Consequently, the throughput of these systems is further reduced because of numerous failed random access attempts and further related inefficiencies.